On your control panel, go to networking. That’s https://cloud.digitalocean.com/networking. Click Domains. Under Add a Domain, type your domain, like something.com / something.blah. From the drop-down list where it says droplet or custom IP address, select your droplet. It should auto-fill. Check the droplet you want and IP address is correct. Hit the Create Record button.
With other options (A is set for you now, along with NS records), you can select CNAME for example, and add (e.g.) “www” as the CNAME pointing to something.com. (note the added period/dot after the domain).
With your registrar, you want to change the DNS records. That’s the registrar where you registered your domain name. Under the list of DNS servers, remove those that already exist, or add them as your options are. Add in ns1.digitalocean.com ns2.digitalocean.com ns3.digitalocean.com and save your DNS server information at your domain registrar’s control panel. It should be quick, but it can be 24/48/72 hours for the changes to propagate around the Internet.
And you’re done, aside from MX records for email for example. You can look that up in Digital Ocean’s FAQ after checking for inbox/email set up.
Hope that helps.